// Copyright (c) 2011 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// This file contains the SdchManager class and the DictionarySet
// nested class.  The manager is responsible for storing all
// SdchDictionarys, and provides access to them through DictionarySet
// objects. A DictionarySet is an object whose lifetime is under the
// control of the consumer. It is a reference to a set of
// dictionaries, and guarantees that none of those dictionaries will
// be destroyed while the DictionarySet reference is alive.

#ifndef NET_BASE_SDCH_MANAGER_H_
#define NET_BASE_SDCH_MANAGER_H_

#include <map>
#include <memory>
#include <set>
#include <string>
#include <vector>

#include "base/macros.h"
#include "base/memory/ref_counted.h"
#include "base/observer_list.h"
#include "base/threading/thread_checker.h"
#include "net/base/net_export.h"
#include "net/base/sdch_dictionary.h"
#include "net/base/sdch_problem_codes.h"

class GURL;

namespace base {
class Value;
}

namespace net {

class SdchObserver;

// Provides global database of differential decompression dictionaries for the
// SDCH filter (processes sdch enconded content).
//
// The SdchManager maintains a collection of memory resident dictionaries. It
// can find a dictionary (based on a server specification of a hash), store a
// dictionary, and make judgements about what URLs can use, set, etc. a
// dictionary.

// These dictionaries are acquired over the net, and include a header
// (containing metadata) as well as a VCDIFF dictionary (for use by a VCDIFF
// module) to decompress data.
//
// A dictionary held by the manager may nonetheless outlive the manager if
// a DictionarySet object refers to it; see below.
class NET_EXPORT SdchManager {
public:
    typedef std::map<std::string,
        scoped_refptr<base::RefCountedData<SdchDictionary>>>
        DictionaryMap;

    // A handle for one or more dictionaries which will keep the dictionaries
    // alive and accessible for the handle's lifetime.
    class NET_EXPORT_PRIVATE DictionarySet {
    public:
        ~DictionarySet();

        // Return a comma separated list of client hashes.
        std::string GetDictionaryClientHashList() const;

        bool Empty() const;

        // Lookup the dictionary contents based on the server hash.  Returns
        // a null pointer if the specified hash is not present in the dictionary
        // set.
        // The pointer is guaranteed to be valid as long as the DictionarySet
        // is alive.
        const std::string* GetDictionaryText(const std::string& server_hash) const;

    private:
        // A DictionarySet may only be constructed by the SdchManager.
        friend class SdchManager;

        DictionarySet();
        void AddDictionary(
            const std::string& server_hash,
            const scoped_refptr<base::RefCountedData<SdchDictionary>>& dictionary);

        DictionaryMap dictionaries_;

        DISALLOW_COPY_AND_ASSIGN(DictionarySet);
    };

    SdchManager();
    ~SdchManager();

    // Clear data (for browser data removal).
    void ClearData();

    // Record stats on various errors.
    static void SdchErrorRecovery(SdchProblemCode problem);

    // Briefly prevent further advertising of SDCH on this domain (if SDCH is
    // enabled). After enough calls to IsInSupportedDomain() the blacklisting
    // will be removed. Additional blacklists take exponentially more calls
    // to IsInSupportedDomain() before the blacklisting is undone.
    // Used when filter errors are found from a given domain, but it is plausible
    // that the cause is temporary (such as application startup, where cached
    // entries are used, but a dictionary is not yet loaded).
    void BlacklistDomain(const GURL& url, SdchProblemCode blacklist_reason);

    // Used when SEVERE filter errors are found from a given domain, to prevent
    // further use of SDCH on that domain.
    void BlacklistDomainForever(const GURL& url,
        SdchProblemCode blacklist_reason);

    // Unit test only, this function resets enabling of sdch, and clears the
    // blacklist.
    void ClearBlacklistings();

    // Unit test only, this function resets the blacklisting count for a domain.
    void ClearDomainBlacklisting(const std::string& domain);

    // Unit test only: indicate how many more times a domain will be blacklisted.
    int BlackListDomainCount(const std::string& domain);

    // Unit test only: Indicate what current blacklist increment is for a domain.
    int BlacklistDomainExponential(const std::string& domain);

    // Check to see if SDCH is enabled (globally), and the given URL is in a
    // supported domain (i.e., not blacklisted, and either the specific supported
    // domain, or all domains were assumed supported). If it is blacklist, reduce
    // by 1 the number of times it will be reported as blacklisted.
    SdchProblemCode IsInSupportedDomain(const GURL& url);

    // Send out appropriate events notifying observers that a Get-Dictionary
    // header has been seen.
    SdchProblemCode OnGetDictionary(const GURL& request_url,
        const GURL& dictionary_url);

    // Send out appropriate events notifying observers that a dictionary
    // was successfully used to decode a request.  Note that this can happen
    // after a dictionary has been deleted from the SdchManager (because
    // DictionarySets retain references to deleted dictionaries).
    void OnDictionaryUsed(const std::string& server_hash);

    // Get a handle to the available dictionaries that might be used
    // for encoding responses for the given URL. The return set will not
    // include expired dictionaries. If no dictionaries
    // are appropriate to use with the target_url, NULL is returned.
    std::unique_ptr<DictionarySet> GetDictionarySet(const GURL& target_url);

    // Get a handle to a specific dictionary, by its server hash, confirming
    // that that specific dictionary is appropriate to use with |target_url|.
    // Expired dictionaries will be returned. If no dictionary with that
    // hash exists that is usable with |target_url|, NULL is returned.
    // If there is a usability problem, |*error_code| is set to the
    // appropriate problem code.
    std::unique_ptr<DictionarySet> GetDictionarySetByHash(
        const GURL& target_url,
        const std::string& server_hash,
        SdchProblemCode* problem_code);

    // Construct the pair of hashes for client and server to identify an SDCH
    // dictionary. This is only made public to facilitate unit testing, but is
    // otherwise private
    static void GenerateHash(const std::string& dictionary_text,
        std::string* client_hash, std::string* server_hash);

    // For Latency testing only, we need to know if we've succeeded in doing a
    // round trip before starting our comparative tests. If ever we encounter
    // problems with SDCH, we opt-out of the test unless/until we perform a
    // complete SDCH decoding.
    bool AllowLatencyExperiment(const GURL& url) const;

    void SetAllowLatencyExperiment(const GURL& url, bool enable);

    std::unique_ptr<base::Value> SdchInfoToValue() const;

    // Add an SDCH dictionary to our list of availible
    // dictionaries. This addition will fail if addition is illegal
    // (data in the dictionary is not acceptable from the
    // dictionary_url; dictionary already added, etc.).
    // If |server_hash| is non-null, returns the server hash that may be
    // used as an argument to GetDictionarySetByHash.
    // Returns SDCH_OK if the addition was successfull, and corresponding error
    // code otherwise.
    SdchProblemCode AddSdchDictionary(const std::string& dictionary_text,
        const GURL& dictionary_url,
        std::string* server_hash_p);

    // Remove an SDCH dictionary
    SdchProblemCode RemoveSdchDictionary(const std::string& server_hash);

    // Registration for events generated by the SDCH subsystem.
    void AddObserver(SdchObserver* observer);
    void RemoveObserver(SdchObserver* observer);

    static std::unique_ptr<DictionarySet> CreateEmptyDictionarySetForTesting();

private:
    struct BlacklistInfo {
        BlacklistInfo()
            : count(0)
            , exponential_count(0)
            , reason(SDCH_OK)
        {
        }

        int count; // # of times to refuse SDCH advertisement.
        int exponential_count; // Current exponential backoff ratchet.
        SdchProblemCode reason; // Why domain was blacklisted.
    };

    typedef std::map<std::string, BlacklistInfo> DomainBlacklistInfo;
    typedef std::set<std::string> ExperimentSet;

    // Determines whether a "Get-Dictionary" header is legal (dictionary
    // url has appropriate relationship to referrer url) in the SDCH
    // protocol. Return SDCH_OK if fetch is legal.
    SdchProblemCode CanFetchDictionary(const GURL& referring_url,
        const GURL& dictionary_url) const;

    // Support SDCH compression, by advertising in headers.
    static bool g_sdch_enabled_;

    DictionaryMap dictionaries_;

    // List domains where decode failures have required disabling sdch.
    DomainBlacklistInfo blacklisted_domains_;

    // List of hostnames for which a latency experiment is allowed (because a
    // round trip test has recently passed).
    ExperimentSet allow_latency_experiment_;

    // Observers that want to be notified of SDCH events.
    // Assert list is empty on destruction since if there is an observer
    // that hasn't removed itself from the list, that observer probably
    // has a reference to the SdchManager.
    base::ObserverList<SdchObserver, true> observers_;

    base::ThreadChecker thread_checker_;

    DISALLOW_COPY_AND_ASSIGN(SdchManager);
};

} // namespace net

#endif // NET_BASE_SDCH_MANAGER_H_
